The present invetion combines advances in lithography and electromagnetic grating theory to produce highly efficient diffractive optical elements on planar dielectric or metallic substrates. The present invention includes the production of highly efficient (greater than 90%) and high quality (with precisions near .lambda./100) diffractive optical elements on planar dielectric or metallic substrates, using very large scale integration (VLSI) techniques which are currently used in the fabrication of integrated circuits (IC).
Diffractive optical elements have developed to the stage of becoming practical components of optical systems. These elements can not only replace conventional refractive and reflective elements but also, in many case, perform functions not even possible with conventional elements. Elements have been made to operate as CO.sub.2 laser beam shapers, laser beam multiplexers, and two-dimensional scanners, all uniquely suited to their application.
A number of independent developments (one in EM-optics theory, another in pattern lithography and a third in large scale dry reactive ion-etching techniques), have made it feasible to generate binary holographic gratings with blaze-like characteristics and near perfect efficiency.
Electromagnetic theory predicts that binary gratings with the proper parameters can achieve a first order diffraction efficiency of nearly 100% over wide wavelength bandwidths and field-of-views. If the periodicity of the grating is on the order of or smaller than the radiation wavelength, all of the orders become evanescent except for the zero and +1st orders. By choosing the depth to period ratio and duty cycle properly, the zero order can be suppressed as well, placing virtually all of the incident radiation into the remaining and 1st diffracted order.
The diffraction efficiency exceeds the limits set by Fourier optics because of the large .lambda./T ratio (T=grating periodicity) which places the diffraction well into the EM-optics domain. Implicit in the Fourier optics assumptions are the Huygens' and Kirchoff's assumptions of scalar diffraction theory where light propagates unperturbed to various portions of the grating surface, then is specularly reflected, constructively or destructively interferes, and imposes the desired phase and amplitude modulation on the reflected wavefront. When the grating periodicity approaches the wavelength of the illumination, the concept, of these specular reflections no longer has any meaning. On these binary gratings operating in the EM dimension gratings all diffraction orders, except the plus-first an zeroth orders, can be made to disappear by diffraction into the substrate material. These suppressed or evanescent orders then will redistribute most of their energy into the remaining radiative order. Therefore, like conventionally ruled echelle-type gratings, where unwanted orders are suppressed by the asymmetry in the grating profile, these gratings behave in a blaze-like (single diffraction order) manner. The sole asymmetry in the use of these binary gratings is in their off-axis illumination. These gratings with their two-level (binary) laminary relief profile can be used with either transmissive or reflective substrate materials, provided the profile depth is chosen appropriately.
The large scale integration advances that allow IC fabrication techniques to be used to produce high efficiency optical elements include the high-resolution and high-accuracy lithographic pattern generation techniques such as projection printing, electron beam an X-ray writing. The improved accuracy and resolution in VLSI pattern generation and device fabrication have made it feasible to computer generate holographic masks with space-bandwidth products of 10.sup.10, or as high as 10.sup.12, half-micron linewidths and 100.ANG. line acuity.
The improvements in dry-etching techniques include ion bombardment and reactive ion etching, which the present invention uses in the production of optical elements. The great effort expanded by the integrated circuit industry has provided the capability to produce binary phase relief patterns with 0.5 .mu.m periodicities and 0.1 .mu.m accuracies over areas with 20 cm or more diameter. All of the above advances are prerequisites to using IC production techniques to fabricate highly efficient and high quality diffractive optical elements.
The present invention incorporates the developments mentioned above into the production of highly efficient diffractive optical elemntes on planar dielectric or metallic substrates. These optical elements operate in the EM domain when the grating spatial frequency (1/T) exceeds the spatial frequency of the illuminating wavefront, i.e., T.ltoreq..mu. and include infrared planar lenses and high speed scanners.
The use of high resolution computer generated lithography makes it possible to manufacture these optical elements (1) by electromagnetic diffraction theory to implement an efficient carrier relief profile, an (2) by use of conventional Fourier optics rules to place a spatial modulation on that carrier. In view of the foregoing discussion, it is apparent that the present invention includes a new method of manufacturing highly efficient diffractive holographic opitcal elements using VLSI techniques normally used in the fabrication of integrated circuits.